3. Communication-assisted voltage regulation

Due to the intermittency of RES and PEVs, the conventional control schemes for OLTC and DGs fail to provide proper voltage regulation. This shortcoming can be compensated using communication-assisted voltage regulation schemes. In the literature, the communication-assisted schemes fall under two approaches: distributed and centralized [3]. Both approaches involve investment in communication links and remote terminal units. The distributed (intelligent) approach is considered to be an expert-based control or model-free approach, which coordinates a variety of voltage control devices with the goal of providing effective and nonoptimal voltage regulation with fewer communication requirements [9]. On the other hand, the centralized approach relies on a central point that monitors the system status and optimizes the operation of voltage control equipment. Typically, a centralized optimization problem is solved to dispatch the reactive power of different voltage control equipment based on (i) load forecasting and (ii) generation monitoring. Several solutions have been proposed in the literature to provide optimal reactive power dispatch for DGs [10–12]. In this section, the role of PEVs in optimal voltage regulation is explained as in [13].

#### 3.1 PEV impact on voltage regulation

Figure 5 represents a simplified multi-feeder distribution network connected to a substation through an OLTC. The test network has a photovoltaic (PV)-based DG and a PEV parking lot, which are connected at different feeder terminals. Following the derivation of (25), the per-unit voltage deviation for both DG and PEV busses can be approximated by

$$\begin{aligned} \Delta \mathbf{V}\_{PV} & \approx (\mathbf{P}\_{PV} - \mathbf{P}\_{L\_1}) \mathbf{R}\_{f\_1} + (\mathbf{Q}\_{PV} - \mathbf{Q}\_{L\_1}) \mathbf{X}\_{f\_1} \\ \Delta \mathbf{V}\_{EV} & \approx -(\mathbf{P}\_{EV} + \mathbf{P}\_{L\_2}) \mathbf{R}\_{f\_2} - \left(\mathbf{Q}\_{EV} + \mathbf{Q}\_{L\_2}\right) \mathbf{X}\_{f\_2} \end{aligned} \tag{26}$$

where PPV, PEV, and PL are DG, PEV, and load active powers, respectively, and QPV, QEV, and QL are DG, PEV, and load reactive powers, respectively.

Equation (26) shows that two worst-case scenarios may occur: (i) overvoltage, when the DG generates its maximum power during light loads and

(ii) undervoltage, during a peak load demand and low DG output. The integration

problem during high PV power generation and peak EV demand, resulting in

For that reason, the power electronic converters that interface DGs and PEVs should be utilized in voltage regulation. The DG can support the voltage regulation through two options: (i) absorbing reactive power and/or (ii) curtailment of active power. The first option is preferred since active power curtailment represents an energy waste. However, the capacity of the DG converter may limit the reactive power support and force the second option. To increase the reactive power support, the interfacing converter of the PEV can be employed to inject its surplus reactive power, thus reducing the DG active power curtailment [13]. A novel optimal coordinated voltage regulation scheme is presented to coordinate PEV, DG, and OLTC to achieve optimal voltage regulation and satisfy the self-objectives of each voltage

As shown in Figure 7, the OLTC is represented by a π-circuit model [14]. The taps are assumed to be at the primary side (high voltage). Subsequently, the OLTC

where Y<sup>T</sup> is the transformer series admittance, a is the turns ratio given in (2), and t denotes the time instant. To take the physical busses into account, (27) can be

> YT <sup>a</sup><sup>2</sup> � YT a


where YOLTC is the OLTC Y-bus admittance matrix, which represents the OLTC

The conventional OLTC controller, shown in Figure 8(a), is modified to emulate an adaptive reference by considering the system's minimum and maximum

ized OLTC controllers (COC) proposed in [13] and shown in Figure 8(b), which

3 5

YT

Vð Þ <sup>0</sup>;<sup>t</sup> I 0 ð Þ 0;t

> 3 7 5

max, respectively. This modification forms the central-

Vð Þ <sup>0</sup>;<sup>t</sup> Vð Þ <sup>1</sup>;<sup>t</sup>

" #

(27)

(28)

" #

excessive tap operation.

Voltage Regulation in Smart Grids

DOI: http://dx.doi.org/10.5772/intechopen.85108

control device.

rewritten as

voltages, that is, Vsys

Figure 7.

61

Equivalent π-circuit model of OLTC.

3.2 OLTC centralized control

secondary voltage and current can be calculated by

Ið Þ <sup>0</sup>;<sup>t</sup> Ið Þ <sup>1</sup>;<sup>t</sup>

admittance in the power flow equations.

minand Vsys

¼

2 6 4

" #

Vð Þ <sup>1</sup>;<sup>t</sup> Ið Þ <sup>1</sup>;<sup>t</sup>

¼

1 <sup>a</sup> � <sup>a</sup> Y<sup>T</sup> 0 �a

2 4

gF þ jb<sup>μ</sup> þ

� YT a

" #

Figure 5. Simplified distribution network with DG and PEVs.

of DGs changes the voltage profile significantly and complicates the voltage regulation. This is due to two reasons: (i) the voltage trend not descending from the substation to the feeder terminal, thereby invalidating the target point (reference) and (ii) the voltage estimation, based on local measurements, becoming inaccurate because of the stochastic power natures of RES and EVs [13]. Moreover, the stochastic power nature of EVs makes the voltage estimation inferior and aggravates the undervoltage problem. Therefore, OLTCs may suffer from wear and tear due to excessive operations. This problem worsens when feeders suffer from overvoltage due to high DG penetration, while others suffer from undervoltage during high demand, such as PEV charging. In this instance, the OLTC will have two contradicting solutions. Increasing the transformer's secondary voltage mitigates the undervoltage problem at the expense of the system's overvoltage and vice versa. Figure 6 shows two daily power profiles for uncontrolled<sup>1</sup> PEV charging demand and a PV-based DG. The PEV demand is generated based on practical arrival/ departure times from the Toronto Parking Authority (TPA), Toronto, Canada. Since the power profiles of commercial parking lots and PV-based DGs naturally coincide, there is a high chance that the system simultaneously suffers from overvoltage and undervoltage. A partial solution for this problem can be realized if a centralized-based controller for the OLTC exploits the system's maximum and minimum voltages. However, this controller may not prevent the OLTC hunting

Figure 6. DG and PEV power profiles.

<sup>1</sup> In uncontrolled charging schemes, PEVs start charging as soon as they are plugged in.
